Heterojunction devices are formed by combining two different semiconductor materials such as crystalline silicon and amorphous silicon. Amorphous silicon is a desirable material to employ because it can be biased to exhibit two distinct states which differ by several orders of magnitude; namely, an "off" state characterized by very low electrical current conductivity, and an "on" state characterized by comparatively high electrical current conductivity. By appropriately controlling the biasing of such material, in the pulsed regime, one may selectively switch the material back and forth between the two states. Controllable, reversible switching between distinct, stable states is a basic requirement of memory cells of the type used in computer memory applications.
The prior art reveals the use of doped amorphous silicon in heterojunction devices such as bipolar transistors which exhibit high current gain: Symons, et al, "Silicon Heterojunction Bipolar Transistors With Amorphous and Microcrystalline Emitters", Solid-State Electronics 30, p. 1143 (1987); and, in solar cells: Carlson, "Amorphous Solar Cells", IEEE Transactions on Electron Devices ED-24,p. 449 (1977). Amorphous silicon heterojunction devices such as electrically programmable read only memories ("EPROM"s) which exhibit irreversible switching behaviour are known: Shacham-Diamond, et al "The Electrical Properties of Ion-Implanted Amorphous Silicon Programmable Element in the Unprogrammed State", IEEE Transactions on Electron Devices ED-37, p. 159 (1990). Two-terminal doped amorphous silicon heterojunction devices capable of exhibiting dynamic, reversible switching behaviour are also known: Chen, et al, "Electrical and Optical Characteristics of an a--Si:H/c--Si Heterojunction Switch", Japanese Journal of Applied Physics 29, p. 1415 (1990). Chen et al have also reported three-terminal devices incorporating amorphous silicon, but have reported investigation of switching behaviour only in respect of two-terminal devices.
The present invention facilitates the fabrication of three-terminal heterojunction bipolar structures utilizing undoped amorphous silicon. A single structure of this type has been found to exhibit static switching behaviour. That is, the structure can be controllably switched between the two conductivity states aforesaid; and, when controllably switched to a particular state, remains in that state until controllably switched to the other state. Conventional static memory devices comprise several inter-coupled transistor structures. The invention accordingly facilitates simplified construction of static memory devices which are less complex than prior art static memory devices.